Lucas-Lehmer test for Mersenne numbers (Scheme)

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(define lucas-lehmer
  (lambda (p)
    (define loop
      (lambda (s M count)
        (if (> count 0)
            (loop (remainder (- (* s s) 2) M) M (- count 1))
            s)))
    (let* ([s 4]
           [M (sub1 (expt 2 p))]
           [result (loop s M (- p 2))])
      (= result 0))))
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